Fabrication of nano-scale temperature sensors and heaters

ABSTRACT

The method for the fabrication of nano scale temperature sensors and nano scale heaters using focused ion beam (FIB) techniques. The process used to deposit metal nano strips to form a sensor is ion beam assisted chemical vapor deposition (CVD). The FIB Ga +  ion beam can be used to decompose W(CO) 6  molecules to deposit a tungsten nano-strip on a suitable substrate. The same substrate can also be used for Pt nano-strip deposition. The precursors for the Pt can be trimethyl platinum (CH 3 ) 3 Pt in the present case. Because of the Ga +  beam used in the deposition, both Pt and W nano-strips can contain a certain percentage of Ga impurities, which we denoted as Pt(Ga) and W(Ga) respectively. Our characterization of the response of this Pt(Ga)/W(Ga) nano scale junction indicates it has a temperature coefficient of approximately 5.4 mV/° C. This is a factor of approximately 130 larger than the conventional K-type thermocouples.

This invention relates to nano scale systems and more particularlyrelates to nano scale temperature sensors and heaters, their method offabrication and apparatus for doing so and claims the benefit ofpriority to U.S. Provisional Application Ser. No. 60/272,155 filed onFeb. 28, 2001.

BACKGROUND AND PRIOR ART

As semiconductor devices continue to decrease in size and more and moreapplications have been found for the micro and nano-devices, there is anurgent need for measuring physical quantities at sub-micrometer or evennanometer size dimension. Temperature is one of the findamental physicalquantities that routinely needs to be measured in order to derive otherthermodynamic quantities such as heat, energy, or specific heatcapacity.

Presently temperature can be measured using a thermocouple, asemiconductor diode, a metallic resistor, a thermister, an infraredthermometry, a near-field thermnometry, and other more exotic methods.Each technique has its own advantages and disadvantages. For example,the thermocouple is extremely simple in design and inexpensive, whilethe semiconductor diode is not suitable for high temperature but usedextensively to measure cryogenic temperature. The spatial resolution ofmost of the above techniques is about 10 microns, except for the nearfield thermometry in which the spatial resolution can be in the order of50 nm. But near field thermometry involves complicated opticalinstrumentation and is very cumbersome. There are several efforts toimprove the spatial resolution of temperature measurements by combiningscanning tunneling microscopy (STM) or atomic force microscopy (AFM)with a small thermocouple or thermistor tip. U.S. Pat. No. 5,581,083 toMajumdar et al, described a method of fabricating nanometer hole on thetip of a scanning probe. The size of the holes define the area ofinteraction of the sensor with the environment. In U.S. Pat. No.5,838,005, Majumdar et al extended the above patent to include usingfocus ion beam for fabricating the nano scale holes. However, it shouldbe noted that the size of their sensors is still limited by thelithographic technique used. In addition, thermoelectricity of certainbulk materials drop drastically due to the constricted geometry of theAFM tip arrangement.

It would be very important to develop a much smaller sensor and aprocess by which the size of said sensor can be controlled to a nanosize as small as 50 nm.

SUMMARY OF THE INVENTION

The first objective of the present invention is to provide a method ofproducing a sensor with sub-micron and/or nanometer dimension.

The second objective of this invention is to provide a method ofproducing a temperature sensor with a very high sensitivity.

The third objective of this invention is to provide a method ofproducing a nano scale heater.

A preferred embodiment of the invention is the preparation of anano-scale sensor comprising the steps of: depositing a first metalnano-strip preferably of tungsten, on an electrical insulator substrateby means of a FIB deposition process; and then depositing a second metalnano-strip preferably platinum, on the same said substrate by mean ofsaid FIB process in a partially overlapping fashion on said first metalto provide a sensing junction as said overlap.

The invention also includes a nano-scale sensing device, preferably athermocoupler and a nano sensor, having the combination of: partiallyoverlapping separate nano-strips of first W and Pt; an electricalinsulator onto which said partially overlap nano-strips are deposited;an output electrode connected to the W nano-strip; and, a second outputelectrode electrically separate from said first electrode is connectedto the Pt nano-strip.

In preferred embodiments, the first metal nano sized strip and thesecond metal nano sized strip each can include a thickness (diameter) ofapproximately 50 nm, and a bi-metal sesning junction therebetween thatcan include a cross-sectional area of approximately 50×50 nm₂.

Further objects and advantages of this invention will be apparent fromthe following detailed description of the presently preferredembodiments which are illustrated schematically in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(a) shows a cross section of an overlapped nano-scale sensorconfiguration of the invention.

FIG. 1(b) shows a cross section of a ball shape junction sensorconfiguration of the invention.

FIG. 1(c) shows a cross section of a point junction sensor configurationof the invention.

FIG. 2 shows experimental setup for deposition of metal strips with Ga⁺focused ion beam.

FIG. 3 shows Nano-scale sensor for testing or generating a temperatureresponse.

FIG. 4 shows a typical K-type thermocouple response to a temperaturechange of 6° C.

FIG. 5 shows the response of nano-scale temperature sensor to a 24° C.temperature change.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the disclosed embodiment of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation.

The method of the invention is based on the discovery that it ispossible to create a nano-scale temperature sensor by a FIB (Focused IonBeam) technique in which the separate nano-strips of W and Pt aredeposited onto a substrate through a FIB deposition process. Thejunction of W and Pt nano-strips formed during the FIB process acts as anano-scale temperature sensor. The diameters of the W and Pt nano-stripswill be on the order of approximately 50 nm, and the cross-sectionalarea of W/Pt junction will be on the order of approximately 50×50 nm²,which will be the smallest temperature sensors that can be made so far.The ends of the nano-strips can be deposited each onto larger conductivesurfaces that would be connected to a sensing unit. Notice that thejunctions can be made from different materials, and can also havedifferent configurations, e.g., meet at a point or overlap layers ofeach of the nano-strips. Pt and W have been employed in this inventionbecause they can easily be deposited using commercially availabledeposition instrumentation.

In a preferred embodiment, we fabricate the sensor contact using afocused ion beam technique FIB. The fabrication of a bi-metal junctionon a suitable substrate will be carried out with a focused ion beamsystem, such as but not limited to the FEI 611 or FEI 200 TEM FIBsystem. For the tungsten nano-strip deposition, a feed gas of tungsten[W(CO)₆] is used as the tungsten precursors for the FIB CVD (chemicalvapor deposition) process. A CVD process is described in U.S. Pat. No.5,510,098 to the same inventor and assignee of the subject inventionwhich is incorporated by reference. The feed gas is admitted into theFIB chamber through a local nozzle. The nozzle is approximately 100 μmfrom the surface during the deposition procedure. W(CO)₆ molecules areabsorbed on the surface, bombarded by the Ga⁺ ion beam, and decomposed.As a result, tungsten metal with approximately the dimension of the ionbeam is deposited onto the substrate. The size of the nano-strip isprimarily limited by the resolution of the FIB system used. After thedeposition of the tungsten nano-strip, the same substrate can be usedfor the platinum nano-strip deposition. The precursors for the Pt metaldeposition will be trimethyl platinum (CH₃)₃Pt in the present case. Morethan one FIB system can be used for the depositions. The FEI 611 has aminimum spot size of approximately 50 nm. The FEI 200TEM FIB system hasa minimum beam spot size of approximately 7 nanometer and can be used totrim the deposition materials or substrates with a approximately 7 nmresolution. The minimum width of the as-deposited metal strip isapproximately 100 nm, but can be trimmed using the Ga⁺ beam after thefinal deposition.

The fabrication process can be used to prepare a heater or a differentsensor. The resistive segment of the heater can be made up ofalternating segments of two different FIB deposited metals. Because thedeposited metals are not pure, but contain some Ga and othercontaminants, the deposited metal will have a resistivity that willpermit heating when an electric current is passed through the metal. Theability of the FIB to mill or deposit at high lateral resolution permitsthe preparation of site specific sensors or heaters. Sensors or heaterscan then be adapted, for example, to micro-electromechanical system(MEMS), even as part of the MEMS fabrication process with the structureof substantially that of FIG. 3.

Reference should now be made to FIGS. 1(a)-(c) that show cross sectionsof three different thermocouple junction configurations, i.e.,overlapped 2, ball shaped 3 and a point 6, of the invention all are ofnano sensor metals noted as tungsten 8 and platinum 9. The overlappingconfigurations can be used as appropriate for the sensors.

Reference should now be made to FIG. 2 that shows a schematic diagram ofthe focused ion beam assisted chemical vapor deposition system used inthe process of making the supported nano-scale temperature sensors andheaters of the invention. During the FIB assisted chemical deposition22, Ga⁺ will be used as the ion beam 24, and W(CO)₆ and (CH₃)₃Pt will beemployed as the gas precursors for W and Pt depositions 26, respectivelyonto the substrate 32. The metal nano-strip deposit can be formed wherethe Ga⁺ ion beam 28 is scanned. Therefore, the different temperaturesensor configurations of the invention can be made through this FIBassisted chemical vapor deposition experimental setup. Finally, thedimensions of the temperature sensors can be trimmed down to thenanometer scale.

Reference should now be made to FIG. 3 that shows the nano-scale sensorapparatus for temperature measurement. This apparatus is positioned onan electrical insulator 32 usually of glass, onto which has beendeposited a conducting pattern that consists of electrodes 36 and 38spaced apart by a very narrow gap 40. A nano strip 42 of tungsten isdeposited in the requisite overlap portion 44 onto nano strip 46 ofplatinum. The data obtained from this apparatus is displayed in FIG. 5.

Reference should now be made to FIG. 4 that shows the actual temperatureresponses of a K-type thermocouple. This data should be compared withFIG. 5 and shows the actual temperature response of the nano scalesensor of the invention.

As noted above, FIG. 4 is the temperature response of a conventionalK-type thermocouple which consists of a Chromel Alumel junction. One cansee from this data that the sensitivity of the K-type thermocouple isapproximately 0.04 m V/degree Centigrade (° C.).

Now if you compare the K-type data of FIG. 4 with the data provided bythe new FIB fabricated nano-sensor of FIG. 5, it is found that thesensitivity of the nano sensor is much higher. In FIG. 5, approximately24° C. temperature change results in a voltage change of approximately125 mV. This is a sensitivity factor approximately 130 times more thanthe conventional thermocouple. Thus, this extraordinarily enhancedsensitivity of the nano sensor of the invention is shown.

The advantage of the invention is that the nano scale dimension of thesensors can be precisely controlled through the FIB deposition process,which can be as small as a few tens of nanometer. This will enablemeasurement of the temperature from the subjects with differentdimensions in a nanometer or a sub-micrometer range. The possible usesof the invention could be microelectronics industries or nanotechnologyindustries. Also the nano-sensor is site specific, meaning that it canbe locate precisely at almost any solid. Furthermore, the nano sensor isless intrusive and can be used in medical applications, e.g., to measurethe temperature of a single cell. Note further that existingthermocouples are much larger (orders of magnitude) than the nano-sensordisclosed herein.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1. A method of making a nano sized sensor comprising the steps of: (a)depositing a first metal nano sized strip on an electrical insulatorsubstrate by a FIB (Focused Ion Beam) deposition process; (b) depositinga second metal nano sized strip on the same said substrate by said FIBprocess in a partially overlapping portion on said first metal nanosized strip, the second metal nano sized strip being formed of adifferent metal material from the first metal nano sized strip, thepartially overlapping portion being selected from one of a ball-shapedportion, and a point shaped configuration portion, wherein the firstmetal nano sized strip and the second metal nano sized strip eachinclude a thickness of approximately 50 nm; and (c) forming a bi-metalsensing junction from the partially overlapping portion between thefirst metal nano sized strip and the second metal nano sized strip,wherein the bi-metal sensing junction includes a cross-sectional area ofapproximately 50×50 nm₂ and the junction is selected from one of aball-shaped portion, and a point shaped configuration portion.
 2. Themethod of claim 1, wherein one of the said first metal-nano sized stripand the second metal nano sized strip is W(tungsten), and another ofsaid first metal nano sized strip and the second metal nano sized stripis Pt(platinum).
 3. The method of claim 2, wherein said sensor is athermocouple with a sensitivity approximately 5.4 mV/degree Centigrade.4. The method of claim 1, further comprising the step of: sensingtemperature at the bi-metal junction, wherein the nano sized sensorfunctions as a thermocouple to sense temperature.
 5. A method of makinga nano sized sensor comprising the steps of: (a) depositing a firstmetal nano sized strip on an electrical insulator substrate by a FIB(Focused Ion Beam) deposition process; (b) depositing a second metalnano sized strip on the same said substrate by said FIB process portionon said first metal nano sized strip, the second metal nano sized stripbeing formed of a different metal material from the first nano sizedstrip, wherein the first metal nano sized strip and the second metalnano sized strip each include a thickness of approximately 50 nm; and(c) forming a ball-shaped portion to create a bi-metal sensing junctionthe portion between the first metal nano sized strip and the secondmetal nano sized strip, wherein the bi-metal includes a cross-sectionalarea of approximately 50×50 nm₂.
 6. The method of claim 5, wherein oneof the first metal nano sized strip and the second nano metal sizedstrip is W(tungsten) and another of the first metal nano sized strip andthe second nano sized strip is Pt(platinum).
 7. The method of claim 6,wherein said sensor is a thermocouple with a sensitivity ofapproximately 5.4 m V/degree Centigrade.
 8. The method of claim 5,further comprising the step of: sensing temperature at the bi-metaljunction, wherein the nano sized sensor functions as a thermocouple. 9.A method of making a nano sized sensor comprising the steps of: (a)depositing a first metal nano sized strip on an electrical insulatorsubstrate by a FIB (Focused Ion Beam) deposition process; (b) depositinga second metal nano sized strip on the same said substrate by said FIBprocess on a portion of said first metal nano sized strip, the secondmetal nano sized strip being formed of a different metal material fromthe first nano sized strip, wherein the first metal nano sized strip andthe second metal nano sized strip each include a thickness ofapproximately 50 nm; and (c) forming a point shaped configurationportion to create a bi-metal sensing junction from the portion betweenthe first metal nano sized strip and the second metal nano sized strip,wherein the bi-metal sensing junction includes a cross-sectional area ofapproximately 50×50 nm².
 10. The method of claim 9, wherein one of thefirst metal nano sized strip and the second nano metal sized strip isW(tungsten) and another of the first metal nano sized strip and thesecond nano sized strip is Pt(platinum).
 11. The method of claim 10,wherein said sensor is a thermocouple with a approximately 5.4 mV/degreeCentigrade.
 12. The method of claim 10, further comprising the step of:sensing temperature at the bi-metal junction, wherein the nano sizedsensor functions as a thermocouple.